High temperature erosion resistant coating and material containing compacted hollow geometric shapes

ABSTRACT

A material system ( 60 ) contains close packed hollow shapes ( 50, 70 ) having a dense wall structure ( 52, 66 ), which are bonded together and which may contain a matrix binder material ( 56 ) between the shapes, where the system has a stable porosity, and is abradable and thermally stable at temperatures up to possibly 1700° C., where such systems are useful in turbine apparatus.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to high temperature, erosionresistant coatings, and more particularly relates to the use of suchcoatings as abradable seals and thermal barrier coatings.

[0003] 2. Background Information

[0004] Most components of combustion turbines are operated at very hightemperatures and often require the use of specialized coatings/insertsto protect underlying supporting materials. These specializedcoating/inserts include thermal barrier coatings (TBCs), disposedopposite the turbine blade tips, as taught in U.S. Pat. No. 5,180,285(Lau).

[0005] Conventional TBCs typically comprise a thin layer of zirconia. Inmany applications, the coatings must be erosion resistant and must alsobe abradable. For example, turbine ring seal segments, which fit withtight tolerances against the tips of turbine blades, must withstanderosion and must also preferentially wear or abrade in order to reducedamage to the turbine blades, and form a tight seal with the turbineblade. Protective coating systems can include several layers including ametallic bond or barrier coating of MCrAlY having an alumina scale and,for example, a columnar yttria stabilized zirconia thermal barrier, astaught in U.S. Pat. No. 4,916,022 (Solfest et al.), which can be furthercoated by an erosion resistant layer of alumina or silicon carbide, astaught by U.S. Pat. No. 5,683,825 (Bruce et al.).

[0006] In U.S. Pat. No. 5,780,146 (Mason et al.), 30 wt. % to 50 wt. %(50 vol. % to 60 vol. %) of hollow alumino silicate or alumina spheresof 400 micrometer to 1800 micrometer diameter, and having a hightemperature capability of approximately 1300° C., were used in analuminum phosphate matrix, for an abradable seal. The seal is used overa ceramic matrix composite shroud segment, which may comprise siliconcarbide fibers in an alumina matrix. However, this invention is limitedin thermal stability due to uncontrolled sphere distribution andcontact, therefore, the matrix controls the thermal stability of systemand limits the temperature of the system to less than 1200° C.

[0007] Fillers have also been used by Naik et al., in U.S. Pat. No.5,064,727. There, abradable stationary seal walls, for jet turbinehousings which seal opposing, rotating rotor blade tips, have a ceramiccore containing from 30 vol. % to 98 vol. % solid ceramic filler, wherethe ceramic fills a honeycomb wall structure. This is then covered witherosion and corrosion resistant outer layer, which is made porous byuniformly dispersed, finely divided filler. The pores can be filled withceramic, metal oxide or carbide materials. Fillers mentioned includehollow ZrO₂.8YO₃ ceramic spheres and solid Al₂O₃,SiC,TiC and BN spheres.

[0008] Other abradable honeycomb structures for use in turbines aretaught in U.S. Pat. No. 4,867,639 (Strangman). There, low meltingfluorides, such as BaF₂, are incorporated into a stabilized zirconia oralumina matrix which, in turn, is used to fill a honeycomb shroud liningmade of, for example, a metal alloy. The filling becomes molten when therotating blade tips rub the shroud, and upon resolidification, improvethe smoothness of the abraded surface. Ainsworth et al., in U.S. Pat.No. 4,639,388, teaches another variation of reinforced ceramic layers,including a honeycomb matrix for use in a turbine as abradable seals.

[0009] In U.S. patent application Ser. No. 09/261,721 (Docket No. RDM97-017-ESCM 283139-00315, Merrill et al., filed on Mar. 3, 1999), ahoneycomb structure having open cells was filled, and optionallyoverlaid, with a material containing hollow ceramic particles embeddedin an interconnected ceramic matrix, to provide a composite thermalbarrier composite coating having superior erosion resistance andabrasion properties for use on combustion turbine components. The hollowparticles were preferably spherical and made of zirconia, alumina,mullite, ceria, YAG or the like, having an average particle size ofabout 200 micrometers (0.2 mm) to 1500 micrometers (1.5 mm). The steadystate erosion rate, grams lost/kg erosive impacting media, of thisfiller was 3.2 g/kg vs. 4.6 to 8.6 g/kg for conventional TBCs. Here, theceramic matrix comprised an interconnected open cell honeycombstructure, binding the hollow spheres together where the hollow sphereswere bonded by a network of aluminum phosphate bridging bonds.

[0010] In U.S. patent application Ser. No. ______ (Docket No.99E7538US-ESCM 283139-00936; Merrill), filed on ______, a vacuumpacking/impregnation method of bonding hollow geometric shapes wasdescribed, to provide abradable, thermally stable seals and the like.Both U.S. patent application Ser. Nos. 09/049,369 (Docket No. T2 97-026,ESCM 283139-00315, Morrison et al., filed on Mar. 27, 1998) and Ser. No.09/049,328 (Docket No. RDM 97-005, ESCM 283139-00374, Merrill, filed onMar. 27, 1998), teach ceramic insulating coatings with improved erosionresistance and macroscopic closed porosity, utilizing hollow oxide-basedspheres which can contact at least 3 or 4 other hollow spheres toprovide improved dimensional stability at temperatures up to about 1600°C. Erosion rate, grams lost/kg erosive impacting media was 4.5 g/kg. and7.5 g/kg.

[0011] However, none of these coatings or seal structures have optimizedabradability with erosion resistance and insulating capability,minimized shrinkability and thermal mismatch, provided constrainedstabilized uniform spherical porosity and adequate flexibility, andoptimized thermal stability for operation substantially up to 1600° C.;all of which characteristics will be required of the next generationhigh temperature turbine TBCs, seals and the like, as well as innon-turbine coating applications. What is needed are high temperaturecoatings, and composites that fill these and other future requirements.

[0012] Also, thermally sprayed structures having hollow spheresco-sprayed to introduce porosity for either abradability or reducedthermal conductivity, are limited to small sphere sizes, typically lessthan 200 microns, for spraying capability. These small spheres tend tomelt in plasma and result in non-spherical pores which are not thermallystable. Such small scale porosity leads to poor erosion resistance.Additionally, thermally sprayed coatings/structures for abradable sealsbased on co-spray of fugitive particles, for example, polyester resinparticles, which are subsequently burned out to leave increasedporosity, results in small, non-spherical porosity and matrix-dominatedproperties which limit thermal stability. The present invention has beendeveloped in view of the foregoing, and to address other deficiencies ofthe prior art.

[0013] Therefore, it is one of the main objects of this invention toprovide a high temperature, erosion resistant coating and material whichis bondable, generally non-shrinking, abradable, flexible, thermallystable up to at least 1600° C., and which has constrained stabilizedporosity and insulating properties, as well as controlled thermalconductivity and thermal expansion properties.

SUMMARY OF THE INVENTION

[0014] These and other objects of the invention are accomplished byproviding a material system, useful as an erosion resistant hightemperature layer, comprising a substantially close packed array ofgenerally contacting, hollow, individually formed geometric shapes,having a coordination number greater than or equal to 1 and having a 70%to 100% dense wall structure, which are bonded together, and whichmaterial system has a constrained stabilized porosity and is abradable,thermally insulating, thermally stable and substantially non-shrinkingat temperatures up to at least 1600° C. Wall thickness greater thanabout 100 micrometers is preferred, in order to provide good erosionresistance. This material provides an optimized combination of physicaland thermal properties needed in the industry but heretofore notattainable, but which will be essential in the future.

[0015] Preferably, the geometric shapes are selected from rigid, hollow,essentially closed ceramic spheres and other similar geometric shapes oflow aspect ratio, less than 10 and preferably less than 5, such ashollow cylinders and the like. The shapes are “individually formed,”defined here as meaning formed separately and then stabilized duringmanufacture, rather than being formed in situ on a substrate etc. Thehollow ceramic shapes have “dense” walls, defined here as having adensity from 70% to 100% of theoretical (0% to 30% porous). Because thegeometric shapes are independently formed, denser wall formationresults, which allows crack deflection and general toughening of thematerial, as well as allowing geometric stability to very hightemperatures approaching 1700° C. Hollow spaces between, for example,one diameter of a first large geometric shapes can be filled withsecond, smaller diameter geometric shapes, to reduce void volume andminimize, consistent with some measure of flexibility, the content ofmatrix ceramic bonds which help bond the shapes together.

[0016] Preferably, there are three dimensional “chains” of hollowshapes, where a substantial number of shapes contact at least 4 to 12preferably 6 to 10 other shapes. This chain or string-like geometryprovides strength and minimizes large void volumes being close to eachother. The material system should have some measure of porousness, atleast 15 vol. % usually up to a maximum 90 vol. %, preferably 40% vol.to 70% vol. for turbine thermal insulating and abradable coatings, andpreferably has some randomness of the contacting shapes in itsstructure. The material system of this invention due to its structure,is also highly friable, while maintaining a low elastic modulus.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017] The invention is further illustrated by the followingnon-limiting drawings, in which:

[0018]FIG. 1 is a cross-section through a gas turbine shroud and otherturbine components in a turbine assembly, showing an abradable seal anda turbine tip, with associated turbine blades and vanes;

[0019]FIG. 2 is a generalized drawing of what one embodiment of thefilled, abradable structure of this invention, containing about 30% vol.to 60 vol. % of generally contacting spheres, having coordinationnumbers of about 3 or 4 to 6, might ideally look like on a microscopiclevel;

[0020]FIG. 3 is a generalized drawing of what the filled, abradablestructure of this invention, containing about 50% vol. to 90 vol. % ofgenerally contacting, substantially close packed spheres, havingcoordination numbers of about 5 to 12, might ideally look like on amicroscopic level;

[0021]FIG. 4, which best shows the invention, is an idealized, enlargedview of one part of one layer of the generally contacting close packedstructure of FIG. 3, but with lower coordination numbers of from about 6to 10, and containing hollow spheres;

[0022]FIG. 5 is a graph of material system density in terms oftheoretical density vs. “t/r” (wall thickness/sphere radius) ratio; and

[0023]FIG. 6 is a graph comparing erosion wear of the material system ofthis invention versus prior art compositions.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0024] Referring now to FIG. 1, a combination of rotatable turbineblades 12

[0025] and stationary turbine vanes 14 are shown in a turbine assemblyapparatus 10 having a turbine casing 16. Turbine shrouds 18 are arrangedcircumferentially around one of the stages of the turbine blades andhave abradable shroud seal layers 20 which cooperate with the turbineblade tip 22 to form a seal 24, shown unproportionally wide for the sakeof clarity. Blade attachment root 26 is also shown as well as supportplate substrate 28 for the seals 20 and potential thermal barrier layer25 on the turbine blade 12. The vanes 14 are stationary and confine andguide hot gases flowing through the associated gas turbine. Thermalbarrier layer 25 could also be used on the vane surfaces. The layer 25is only shown on the edge, but can and usually would cover the entiresurface of the blade 12 and the vane 14.

[0026] In order to improve performance (thermodynamic efficiency andpower output), thermal barrier layers have been applied to cooledturbine components to reduce the amount of cooling air, etc. required.Recently, to improve sealing effectiveness, the seal leakage area 24 hasbeen reduced by having the blade tip insert 22 contact and abrade theshroud seal 22, to provide individual, tight seals between each seal andits corresponding turbine blade. In this invention the shroud seal, andmost other seal or wear areas in the turbine apparatus can be made ofthe erosion resistant, highly filled, high temperature material of thisinvention, utilizing from about 20 vol. % to 85 vol. % of individuallyformed, close packed, hollow geometric shapes having walls over 70%dense. This same coating is also useful as a thermal barrier coating forblades, vanes, combustors and other like objects in a gas turbine enginewhich currently require active cooling.

[0027] In FIGS. 2 and 3 the spheres used in this invention are shown asthree dimensional chain like bodies, but it is understood that they arehollow shapes, more clearly shown in FIG. 4. Referring to FIG. 2, oneexample of a material of this invention is shown as 32, in idealizedform, as a segment within the dotted lines. First layer with spheres Ais somewhat discontinuous, although most spheres contact at least twoother spheres, with a substantial number, that is about 70% contactingthree or four other spheres, which is apparent in locations where thesecond layer spheres B are shown. Second layer spheres, B are alsodiscontinuous, but with substantial contact with first layer spheres.Only one-third layer, black sphere C is shown, for the sake of clarity.The matrix spaces 34 provide moderate porosity in addition to the closedporosity inherent within the hollow geometric shapes. This interstitialporosity is of secondary importance to the overall wear and thermalproperties of the structure, but can be critical to the mechanicalbehavior—especially at the higher porosity range. The erosionperformance is achieved through the use of dense-walled geometric shapeswhich are resistant to small particle erosion. Abradability is likewiseachieved through the introduction of coarse (greater than 100 microns),closed porosity. The simultaneous optimization of both erosionresistance and abradability is achieved through judicious selection ofthe values and scale of closed porosity, hollow sphere size and wallthickness. The contribution of the matrix 34 is primarily in themechanical integrity of the structure (strength and elastic constants).

[0028] Another example of the material of this invention, shown ideallyin FIG. 3, is a close packed array 40, within the segment enclosed bythe dotted lines, having generally contacting, mostly continuous, firstlayer A, mostly continuous second layer B, shown here for sake ofsimplicity as three clusters, and a mostly continuous top layer, hereshown as only two black spheres C, again for clarity. This embodimenthas almost universal chain formation in three dimensions among thespheres, providing a high level of constrained stabilized uniformspherical porosity. That is, the chains prevent collapse and substantialshrinking of the system at temperatures approaching 1700° C. Even thoughmost of the layer B and top layer C spheres are not shown in FIG. 3, itis easy to see how this invention's 50 vol. % to 90 vol. % hollow shape,here hollow sphere configuration, allows minimizing the binder matrixspaces 34. Although the material of this invention is capable of veryhigh temperature use, there are many other useful applications at lowertemperatures, in the range of 1200° C. to 1500° C., so that the mainrange of applications is from about 1200° C. to 1600° C.

[0029] By “substantially close packed” is mean the type array shown inFIGS. 2, 3 and 4, where the coordination number, defined previously, isgreater than or equal to 1. However, some randomness is desired, formaximum fracture toughness, strength and flexibility. FIG. 3 shows twosets of sites for the spheres C of the third layer. A staggered layeringas shown by line 44 among spheres A, B, and C, provides a hexagonalclose packed array partly shown. A straight layering as shown by line 46among spheres A, B, and C provides a face centered cubic array.

[0030] Referring now to FIG. 4, which most clearly illustrates oneembodiment of the invention, hollow spheres 50, having a first size andhaving walls 52, are shown, substantially close packed with, optionalsmaller, embedded, hollow shapes, such as hollow spheres 54 having asecond smaller size, as well as matrix binder material 56 disposedbetween the larger hollow spheres 50. Voids 58 are also shown. Thehollow ceramic spheres 50 are manufactured such that the sphere wallsare about 70% to 100% of theoretical density preferably near 90% to 100%of theoretical density (0% to 10% porous). For good erosion resistance,the wall thickness is preferably between about 100 micrometers to 400micrometers, depending on sphere diameter. The hollow ceramic spheresuniquely control the dimensional stability of the material system 60 andinhibit gross volumetric shrinkage due to sintering of thematrix-regardless of the matrix binder material selected. The hollowspheres are also critical in establishing the unique macro- andmicrostructure of the material 60 and in controlling its uniquebehavior, both thermally and mechanically. The level of closedmacroscopic porosity within the material system 60 is defined by theoverall size of the hollow ceramic spheres, the wall thickness of thespheres, and their packing arrangement within the structure. The binderthat interstitially bonds the hollow ceramic spheres together may alsocontribute a secondary role to these properties depending on themechanical and thermal properties and the amounts of binder materialused. The structure of the material system 60 imparts uniquecombinations of high temperature properties including, for gas turbineapplications, excellent erosion resistance, insulating properties, andabradability in an optimized combination not achieved by conventionalmeans.

[0031] The material system 60, shown in FIG. 4, is manufactured withclosed macroscopic porosity via the use of hollow ceramic spheres (orsimilar geometric shapes) of size ranges between 200 micrometersdiameter to 5000 micrometers diameter, which are independently formedand subsequently bonded together to form a macroscopic infrastructure.The bulk density of these spheres 50 themselves is anywhere between 0.10and 0.90 of theoretical, that is 10% to 90%, taking into account theinternal porosity, and depending upon the needs for each hightemperature application. The close-packed, contiguous structure definesand controls the thermal and mechanical properties of the system 60 andmaintains dimensional stability of the structure. Coordination numbersfor the sphere packing can range from 1 to 14, but preferably, thecoordination number of substantially all of the hollow shapes is from 5to 12, and even more ideally from 5 to 8 for gas turbine coatingapplications. As mentioned previously, some degree of randomness ispreferred for maximum fracture toughness and strength. However,idealized packing arrangements and high coordination numbers in the 8 to14 range, as shown in FIG. 3, may be preferred for stiffness-drivenapplications such as cores for sandwich structures.

[0032] It is critical that the spheres 50 be separately formed andstabilized vs. formed in-situ. The individually formed and bondedspheres impart unique properties to the material system 60. Contactbetween dense-walled spheres is not overly intimate due to the lack ofsintering between spheres at processing and applicationtemperatures—thus allowing crack deflection and some degree oftoughening of the material. Forming the spheres separately allows thestabilization of the spheres to a much higher temperature than otherwisepossible. In-situ formed voids, inevitable in most coating operations,are generally interconnected, thus increasing the overall size ofcritical defects and weakening the structure. By using independentlyformed hollow spheres, substantially all large voids 50 within thehollow spheres are separated by at least 2× the sphere wall thickness,as shown, for example, at point 62 in FIG. 4. It is also desirable thatsmall voids 58 not be interconnected, but separated, as at point 62.

[0033] The ceramic hollow spheres such as 50 in FIG. 4 will typicallyhave a wall thickness-to-radius ratio (“t/r” ratio) of between 0.05 to0.50. Depending upon the application, this t/r ratio can vary, forexample: for lightweight insulation where erosion is not critical (or isaccommodated), t/r in the lower end of this range is desirable; forinsulating or abradable coatings, that is, in a gas turbine, whereerosion is a concern, t/r between 0.1-0.4 is preferable; for very strongor erosion resistant materials, the higher end of the t/r range isdesirable.

[0034] For many applications, the absolute value of the wall thicknessis critical to achieving adequate erosion resistance. Therefore, the t/rand overall diameter of the spheres must be considered. For example, ifa 200 micrometer wall thickness is required for erosion resistance, andthe overall bulk density of the material system is defined byabradability requirements, that is, about 60%, this helps narrow downthe range of sphere sizes possible (other variables include spherepacking density and matrix density). Wall thickness will generally rangefrom about 0.05 mm (50 micrometers) to 0.5 mm (500 micrometers).

[0035] The shape of the hollow particles is ideally spherical, for easeof manufacture and for attaining isotropic properties. However, othersimilar shapes can be readily made and incorporated in like manner andperhaps introduce certain optimized properties, for example, the shapescan be hollow elongated spheroids, or needlelike shapes either randomlyoriented or preferentially oriented parallel or perpendicular to thesubstrate surface and having aspect ratios less than 5-to-1 and lengthsbetween 200 micrometers and 5000 micrometers. Hollow ellipses or otherirregular spheroids can be made and indeed are within the normal rangeof sphere manufacture. Mixtures of spheres, spheroids, low aspect ratiocylinders and other shapes are also natural extensions of this inventionand are conceived herein, in fact, at least one manufacturing processfor hollow spheres also produces hollow, long needle structures.

[0036] Overall bulk density of the entire material system—including thesphere density, their packing arrangement, and the matrix/filler/bindermaterial—is generally in the range of 0.10 to 0.80, that is, 10% to 80%of theoretical density, depending on the application. For the example ofturbine engine coatings for either insulation or abradability (or both),the range of overall density of the material system and its makeup canvary over a wide range. By controlling the bulk density, that is, spheresize, sphere wall thickness and binder density) of the material system,the properties can be optimized for erosion resistance and/orabradability, as shown in FIG. 5, which is, a graph of material systemdensity in terms of theoretical density vs. t/r. The best range forabradable coatings is 30% to 80% theoretical density. The best range forerosion resistance is from about a 0.3 to 0.5 t/r within the 30% to 80%range, while the best range for abradable lightweight coatings is fromabout a 0.1 to 0.3 t/r within the 30% to 80% range, where in all cases“BCC” means body centered cubic packing. Curve 1 is 52% cubic packingand 100% dense matrix; Curve 2 is 52% cubic packing and no matrix; Curve3 is 74% body centered cubic packing and 100% dense matrix; and Curve 4is 74% body centered cubic packing and no matrix. The most preferredcombination of bulk density plus t/r for both abradability, erosionresistance and insulating characteristics is 30% to 80% bulk densityplus a t/r ratio between 0.15 and 0.45.

[0037] Note that while FIGS. 2, 3 and 4 show cases of regular packing ofuniformly sized spheres, the packing of the spheres is not limited toeither form or packing nor is it limited to uniformly sized spheres, noris it limited to any kind of regularity of structure (random packing ismore the norm and is allowable so long as the at least substantially“close-packing” criteria is met). Non-uniform sphere sizing may bedesirable to achieve higher sphere packing densities.

[0038] Sphere walls must be over 70% dense, but are preferably neartheoretical density (90% to 100% of theoretical density) to maintainthermal stability over the widest range of temperatures. The highdensity of the sphere walls imparts excellent erosion resistance andcontrols the thermal and mechanical behavior of the system. Themanufacturing temperature of the hollow spheres is well in excess of theintended use temperature of the material system, for example, mullite(3Al₂O₃∃2SiO₂) hollow spheres can be manufactured at 1750° C., renderingthem inert and stable in a material system operating indefinitely at1600° C. to 1700° C. Separately forming and stabilizing the spheres tohigh temperatures ensures the thermal and dimensional stability of thesystem at future high temperature operating ranges up to 1700° C. andpossibly beyond. The hollow ceramic spheres, rods, etc. are bondedtogether interstitially by a ceramic matrix material to form acontiguous and self-supporting structure. The matrix material itself andthe interconnected network of spheres both form contiguous structures inthe claimed material. The volume content of the matrix material can varyfrom near zero to completely filling in the interstitial space betweenthe hollow shapes. Preferably the matrix constitutes a minimum of 10% ofthe interstitial space between the hollow shapes for all coordinationnumbers. The matrix content and density are tailored to achieve thedesired properties for the specific application, taking into account:the desired bond strength between hollow shapes; the overall bulkdensity required for abradability purposes; the individual and packingdensities of the hollow shapes; permeability requirements; overallmechanical strength of the system; overall thermal conductivityproperties desired; and mass considerations (for example, for flightapplications). The matrix may or may not contain filler or reinforcingmedia, including but not limited to, smaller hollow spheres or othergeometric shapes, powders, particles, platelets, and whiskers or choppedfibers or other discontinuous fibrous materials.

[0039] In the case of thick-walled shapes, where t/r is greater than0.25, it may be advantageous to minimize the amount of matrix in thesystem—so long as the bond strength and other criteria are met. A verystrong structure, particularly in compression, can be achieved with verylittle matrix addition. In the case of thin walled shapes (t/r less than0.25), particularly at the higher coordination number ranges, it may beadvantageous to maximize the amount and density of the continuous matrixphase to increase erosion resistance while maintaining low overallcomposite density, light weight, and low thermal conductivity.”

[0040] The composition of the ceramic hollow shapes can be any oxide ornon-oxide ceramic material including (but not limited to) those shown inTable 1 below: TABLE 1 OXIDES CARBIDES NITRIDES Alumina, Silica SiC, WC,NbC Si₃N₄, TiN Mullite, Zirconia TaC, HfC, ZrC SiCN Zircon, YAG, YttriaTiC Ceria, Hafnia, Beryllia

[0041] The ceramic matrix material may be either of an oxide based or anon-oxide based composition, including (but not limited to) thecompositions also shown in Table 1 above.

[0042] Typically, high temperature, stable ceramic particulate fillermaterials are used in the binder system for the material system. Thepurpose of these fillers may be to add density to the matrix (withoutnecessarily adding strength), add strength to the matrix, add toughnessto the matrix, either via reinforcing or residual stress management, orimprove cost savings. Typically, the particulate material in theslurry-based binder may be of the composition but not limited to thosein Table 2 below, and typical binders are listed in Table 3 below. TABLE2 TABLE 3 PARTICULATE BINDER Mullite Aluminosilicate/AluminumphosphateAlumina Aluminumphosphate/ Aluminumoxycarbide ZirconiaAluminumorthophosphate Hafnia Aluminumorthophosphate YttriaAluminumorthophosphate Yttrium Aluminum Aluminumorthophosphate Garnet(YAG) Ceria Aluminumorthophosphate Silicon Carbide or PolycarbosilaneSilicon Nitride Hollow Shapes of the All Binders Listed Above Above

[0043] Any of a number of existing or conceivable methods of makinghollow ceramic spheres can be used to create the spheres used inmaterial system. A few commercial sources exist for macroscopic hollowspheres (>200 micrometers)—including Keith Ceramics Ltd (UK) and CeramicFillers Inc. (U.S.A.). These sources offer spheres made via traditionalsol-gel or slurry processing routes and are primarily oxide-basedmaterials. However, virtually any process used to form ceramics can beenvisioned for creating hollow ceramic spheres, for example, polymerprecursor coating of fugitive spheres, plasma spraying or reactionforming. In many processes, a certain amount of hollow needle structuresare necessarily produced which with filtering can be used separately asthe geometric shape of the erosion resistant coating of this invention.

[0044] The spaces in the material system matrix may be filled in manyways, including (but not limited to) ceramic processing methods wherethe ceramic matrix material or binder consists of slurry or liquid basedceramic binder, or particulate materials may be mixed with a liquidbinder to form a slurry, the viscosity of which is controlled to providesuitable characteristics. In the latter instance, the liquid binder canconsist of ceramic bonding agent in solution; for example, the liquidbinder may be aluminum orthophosphate solution, alumina or mullite orsilica sol or aluminum hydroxyl chloride. A typical composition ofslurry based ceramic matrix could consist of the following constituents:1 part by wt. sintered mullite powder (25 micrometers average particlesize), and 0.858 part by wt. of 50% aqueous solution aluminumorthophosphate. The ceramic slurry could be premixed with the ceramichollow spheres. The hollow ceramic spheres would pack down duringdrying. The contiguously packed ceramic hollow spheres could be fired attemperatures between 600° C. and 1600° C. In the case of the aluminumorthophosphate solution mentioned above, the transformation of aluminumphosphate to alumina will be proportional to the firing temperature. Thehigher the firing temperature, the greater the volume % of alumina.Alumina provides solid state bonding of the particulate mulliteparticles in the absence of aluminum phosphate.

[0045] As an alternative to slurry based ceramic binder, a liquid basedbinder may be used. The liquid ceramic binder may typically be anaqueous solution of aluminum phosphate or aluminum hydroxyl chloride ora sol such as mullite, alumina or zirconia based. The liquid binder maytypically coat each sphere with a thin coating that, when fired willbond the ceramic hollow spheres at the points of contact or contiguity.Such material may be modified by a secondary slurry based filling stepused to increase the overall bulk density and properties of the finalmaterial by filling the interstitial spaces available between thecontiguously, pre-bonded spheres. As an alternative to slurry based orliquid based ceramic binders, other manufacturing processes may be used,which include but are not limited to polymer precursor impregnation,reaction forming, directed metal oxidation, air plasma spray, chemicalvapor impregnation, and physical vapor deposition of ceramic precursorgases.

[0046] All of these materials preferably have erosion resistance and afair degree of porosity. Erosion is a complex phenomenon which is elatedto material hardness, fracture resistance, grain size, impactingparticle energy, etc. For a given material system, erosion resistance isinversely proportional to the porosity in the material. This is truebecause typical means to introduce porosity yields fine scaleinterconnected pores in materials with dramatically reducedmicrohardness and fracture resistance-thus poor erosion resistance. Aunique aspect of the material system of this invention is the control ofmacroscopic porosity through the use of dense-walled hollow spheres.Closed porosity not only results in superior erosion resistance, butalso allows a more optimum combination of abradability and erosionresistance. The dense sphere walls (high local hardness) offerprotection from erosion while the macroporosity of the system maintainsdesirable insulation and abradability characteristics.

[0047] High levels of porosity can be achieved with the material systemof this invention without sacrificing structural integrity or erosionresistance. High porosity yields low thermal conductivity, that is highinsulation effect, which is vital for the thermal protection of metallicturbine engine components—many of which operate at temperaturesexceeding their melting point. Thermal protection of coated substratesin gas turbine environments is also crucial to their success, and allowsthe use of currently available, moderate temperature capabilitysubstrates in very high temperature environments while still reducingdramatically the required cooling air. Low thermal conductivity isachieved with high porosity levels in this material system withoutsacrificing erosion resistance. Abradability is achieved in thismaterial system by incorporating high levels of porosity withoutsacrificing erosion resistance. Optimized abradable behavior is possibledue to the erosion resistance margin offered by this material systemstructure. These bodies/coatings can be formed having high porosities,yet good chain structure, which were previously unachievable viaconventional means due to the corresponding, unacceptable loss oferosion resistance. The key parameters for truly abradable materials areoverall level of porosity (for a given composition) and “friability”,defined as the ability of abraded particles to be removed from thesurface, thus preventing densification of the subsurface. The materialsystems described, particularly those such as shown in FIG. 3, providesfor both qualities by its relatively high porosity levels and use ofceramic spheres for friability.

[0048] Low elastic modulus is achieved by these material systemsrelative to coatings of similar thermal properties. This is due to theability to achieve high porosity levels without the detrimentalattributes normally associated with highly porous materials. Low modulusis crucial to limiting thermally-induced stress not only in thesematerial systems themselves, but also in any underlying substratematerial. These material systems can be an insulating coating formoderate strength substrates for gas turbine applications. Minimizingstress in the substrate is crucial to achieving successful designs withthe required lifetimes. These material systems are uniquely capable ofproviding good thermal protection without introducing undue stress inthe substrate.

[0049] The properties of the material systems of this invention can betailored over a broad range to match the behavior of matingmaterials—including substrates in coating applications. The thermalexpansion behavior of these material systems is predominantly that ofthe spheres used in the structure, with a secondary effect from thematrix. Spheres form a contiguous network and thus dictate the grossexpansion of these material systems. Optimized thermal expansion can beachieved through sphere composition selection or through a mixture ofdifferent sphere compositions, for example, the thermal expansion of themullite based material system is only about 0.6% at 1000° C. Inaddition, for other applications these material systems also have uniqueproperties of virtually unlimited compositional variations, shapeforming capabilities, electrical properties, being lightweight, havinghigh specific strength and stiffness, and large part fabricationpotential.

[0050] The original and most important application of the materialsystems of FIG. 2, as previously described, is for insulation ofstructural ceramic and superalloy materials of gas turbine components.There are, however, many other potential uses of these material systems.A unique aspect of these material systems is the ability to form thematerial system in close collaboration with other materials to which itmay need to be attached. For example, in the application to ceramicsubstrates for insulation protection, the following processing methodsare possible: this material system can be coated on a ceramic orcomposite substrate through conventional casting processes and co-firedwith the substrate, or a partially-densified material system can bedeposited or cast onto a substrate which has not been fully densified,where the combined structure can then be processed through the remainderof the composite substrate process, thus integrally joining the twomaterials. This material system can also be envisioned as a low cost,lightweight, and strong core material for composite sandwich structureswhere mass-specific properties are desirable. Herein, the materialsystems could be co-processed with the composite skins, resulting in lowcost manufacture and superior structure properties.

[0051] Other applications for the material system of this invention mayinclude application as combustor liners (can, annular, and can-annularconfigurations), transitions, transition ducts, static airfoils andplatforms (vanes), seals (both blade tip and interstage), and virtuallyany static surface exposed to a hot gas path. Aerospace applicationsinclude aircraft hot gas (engine exhaust) impinged structures andsurfaces; thermal protection systems (TPS) for aerospace vehicles(hypersonic or re-entry protection); stiff, lightweight panels orstructures for space systems (satellites, vehicles, stations, etc.),etc. Diesel or other ICE applications include cylinder liners, exhaustports, header liners, etc.

EXAMPLE

[0052]FIG. 6 illustrates the unique erosion properties offered by thecomposite coating system described herein. The erosion resistance for apreferred embodiment of the present invention is compared to literaturedata for similar ceramic material systems. The materials compared are asfollows: Curve 1 is a generic erosion model derived for homogeneousceramic materials with varying degrees of porosity. It is based on amechanistic model of cracking in ceramics. The parameters used in thismodel (impacting particle size and velocity and erosion target materialproperties) are representative of the erosion testing for each sampleevaluated. Since the wear rates are normalized for each sample (comparedto fully dense versions of the same material), this allows comparison ofmaterials tested under different conditions. Samples 1 and 3 were testedunder identical conditions (using 100 micrometer alumina particles at900 ft/sec velocity, impacting at 15° angle).

[0053] Sample 1 was a conventional thermal barrier coating ofyttria-stabilized-zirconia composition applied via plasma spraying.Porosity was introduced via control of spraying parameters and throughco-deposition of fugitive phases (that is, polyester particles). Sample2 was a sintered alumina body formed through cold pressing of powders toa predetermined “green” density followed by sintering at hightemperatures. Porosity was controlled by varying the green density ofthe powder compact prior to sintering.

[0054] Sample 3 was the preferred embodiment of the present inventionand consisted of hollow multi spheres of t/r about 0.3, a particlepacking coordination number of 4 to 6, and a matrix of Mulliteparticle-filled alumina having 50% porosity (fills 50% of the hollowparticle interstitial space). The overall bulk density of the resultingcomposite structure was about 50%. The erosive wear rate of this systemas measured by particle impact testing was greater than anorder-of-magnitude lower than the theoretical model predicts for a givenporosity level. Moreover, the wear rate of the inventive sample at 50%porosity was lower than that of the comparable materials at half thatlevel of porosity. This erosion resistance is achieved due to thecomposite structure and the combination of hard, thick, dense-walledspheres while maintaining high gross levels of porosity.

[0055] The implication of these erosion results is that improvedinsulating and abradability properties can be achieved in the currentinvention though controlled introduction of porosity without the normalcorresponding increase in erosion rate. This combination of propertiesis unique and heretofore unachievable by conventional means.

1-20. (canceled)
 21. A high temperature thermal barrier coating materialfor a turbine component, comprising: a three-dimensional array ofgenerally contacting ceramic geometric shapes having a packing densityof 20% to 85%, the ceramic shapes having a wall structure density of 70%to 100% and a wall thickness of 50 to 500 micrometers; and a binderdisposed within the array and among the ceramic shapes to bind theceramic shapes together within the array, wherein the thermal barriercoating material is thermally stable at temperatures up to 1600° C. 22.The coating material of claim 21, wherein the turbine component isselected from the group consisting of: blade, vane, transition,combustor, and seal.
 23. The coating material of claim 21, wherein theturbine component is in a combustion turbine assembly.
 24. The coatingmaterial of claim 21, wherein the packing density is 35 to 65%.
 25. Thecoating material of claim 21, wherein the wall structure density is 85%to 100%.
 26. The coating material of claim 21, wherein the wallthickness is 100 to 400 micrometers.
 27. The coating material of claim21, wherein the geometric shapes have an aspect ratio of less than5-to-1.
 28. The coating material of claim 21, wherein the geometricshapes are spherical.
 29. The coating material of claim 28, wherein thewall-thickness-to-radius ratio is 0.05 to 0.5.
 30. The coating materialof claim 21, wherein the binder is ceramic.
 31. The coating material ofclaim 30, wherein the ceramic binder is less dense than the ceramicshapes.
 32. The coating material of claim 21, wherein the binderphysically adheres to the ceramic shapes to bind the ceramic shapestogether within the array.
 33. A high temperature material systemadapted for use on a turbine component, comprising: a three-dimensionalarray of generally contacting ceramic geometric shapes having a packingdensity of 20% to 85%, the ceramic shapes having a wall structuredensity of 70% to 100% and a wall thickness of 50 to 500 micrometers;and a filler disposed within the array and among the ceramic shapes tobind the ceramic shapes together within the array, wherein the materialsystem is thermally stable at temperatures up to 1600° C.
 34. Thematerial system of claim 33, wherein the turbine component is selectedfrom the group consisting of: blade, vane, transition, combustor, andseal.
 35. The coating material of claim 33, wherein the turbinecomponent is in a combustion turbine assembly.
 36. The coating materialof claim 33, wherein the filler is ceramic.
 37. The coating material ofclaim 36, wherein the ceramic filler is less dense than the ceramicshapes.
 38. The coating material of claim 33, wherein the fillerphysically compresses the ceramic shapes to bind the ceramic shapestogether within the array.
 39. A high temperature resistant turbinecomponent, comprising: a component selected from the group consistingof: blade, vane, transition, combustor, and seal; and a high temperaturecoating applied to the component, the coating comprising: athree-dimensional array of generally contacting ceramic geometric shapeshaving a packing density of 20% to 85%, the ceramic shapes having a wallstructure density of 70% to 100% and a wall thickness of 50 to 500micrometers; and a binder disposed within the array and among theceramic shapes to bind the ceramic shapes together within the array. 40.The turbine component of claim 38, wherein the coating covers the entiresurface of the component.